A fuel rod for a nuclear reactor fuel bundle includes a fuel tube formed of a zirconium alloy that houses a plurality of uranium fuel pellets. The fuel tube is sealed by upper and lower end plugs welded at ends thereof. In the manufacturing process, the lower end plug is first welded to the fuel tube. The uranium fuel pellets, with varying enrichments, are loaded into the fuel tube, and a plenum spring is inserted on top of the fuel column. The fuel tube is pressurized to a specified internal pressure, and an upper end plug is welded onto the open end of the fuel tube.
The internal fuel rod gas is typically hyperbaric helium to increase the heat transfer performance of the rod in the reactor. This requires the final (upper end plug) weld to be made under hyperbaric helium conditions. The tungsten-inert gas (TIG) weld process is normally used for fuel rod welding; however, TIG weld arcs are unstable and difficult to control in a hyperbaric helium environment.
In the conventional process, the upper end plug is welded to the fuel cladding (seam weld) in one atmosphere of helium, and the fuel rod is then pressurized to the helium design pressure through a small pressurization hole in the upper end plug. Once a stable helium environment is obtained, the pressurization hole is spot welded closed using a TIG weld process (seal weld). The welding of the pressurization hole is of such short duration and simplicity that the high pressure TIG instability has little or no negative consequence.
Although this approach solves the arc instability of the TIG process in hyperbaric helium, the final end plug design is more complex and costly to develop. The process also requires additional inspections, first to assure that the pressurization hole is open and accurately dimensioned to allow sufficient gas flow, and second to assure that the spot weld closed the pressurization hole and provided sufficient-weld thickness to withstand in-reactor operating conditions.
Furthermore, the use of tungsten electrodes risks the possibility of tungsten contamination of the welds. This problem potentially can occur on both the first (lower end plug) and final (upper end plug) weld. Sufficient levels of tungsten contamination in the weld joint can cause chemical corrosion of the weld in the reactor, resulting in a failed fuel rod and costly operating procedures.
Attempts have been made to utilize laser seam welding in an effort to obviate the drawbacks associated with the two-step TIG weld process. In the previous laser seam welding process, welding was attempted with a CO.sub.2 laser at hyperbaric pressure. The process, however, suffered numerous drawbacks, which rendered the process commercially unacceptable. In particular, soot evolved during the welding process that occluded the optics and caused absorption of the laser beam, cracking the optics. Moreover, the welding process was comprised of a pulsed mode seam weld followed by a continuous wave (CW) resurface weld. The pulsed mode welding created the fuel rod cladding to end plug joint. The CW mode resurface welds smoothed the surface of the weld to allow for automatic ultrasonic (compression wave) inspection for weld integrity. During the beam mode change from pulsed to CW, the focal length of the CO.sub.2 laser was not consistent and required constant attention to achieve acceptable process yields. In addition, ionic plasma formed over the weld joints and was suspected of causing laser beam reflections back up into the optics train, which had potentially damaging effects.